The wideband mobile communication technologies, such as Long Term Evolution-Time Division Duplex (LTE-TDD), Mobile WiMAX/TDD, Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) and Femtocell, etc, have introduced high requirements on clock synchronization. The clock synchronization includes not only frequency synchronization but phase/time synchronization. The general requirement on the air interface is a frequency accuracy of 50 ppb (parts per billion) and a phase/time accuracy of the order of 1 μsec. For example, the phase accuracy is ±3 μsec for a CDMA2000 system, ±5 μsec for a LTE-TDD large cell and ±1.5 μsec for a LTE-TDD small cell.
There are various types of synchronization technologies, including time related and frequency related synchronization protocols. An example of time related synchronization protocol is the IEEE 1588 standard, also known as “Precision Clock Synchronization Protocol for Networked Measurement and Control Systems” or “PTP” for short, which is used for phase/time synchronization. Sync Ethernet (SyncE) is a kind of frequency related synchronization protocol and is used for frequency synchronization over the Ethernet link. Both PTP and SyncE are based-on a master-slave mechanism. That is, a downstream node obtains a reference clock from its upstream node, and a master-slave clock relationship is formed between the nodes.
FIG. 1 illustrates a schematic network 100 synchronized by e.g. PTP/SyncE. The network 100 includes nodes 101, 102, 103, 104, 105 and 106. The node 101 obtains a reference clock from a packet master clock capable of providing a reliable Primary Reference Clock (PRC) such as an atomic clock or GPS disciplined oscillator. The port of the node 101 that connects to the packet master clock is indicated as “slave” (S). Then the node 102 obtains the reference clock from the node 101 according to PTP or SyncE. The node 101 becomes the master clock node of the node 102 and the node 102 becomes the slave clock node of the node 101. As shown in FIG. 1, to point out the clock distribution path in the network, the port of the node 101 that connects to the node 102 is indicated as “master” (M), and the port of the node 102 that connects to the node 101 is indicated as S. The clock is in turn distributed to other nodes 103-106 and all the nodes in the network are synchronized. As shown in FIG. 1, the link between the node 101 and the node 106 is blocked to avoid timing loop on the network. The two ports on the link between the node 101 and the node 106 are both indicated as M, since no reference clock is obtained from these ports.
However, synchronization of a node may be lost due to change of the network topology or link/node failure. Assume that the topology of the network 100 is changed such that the link between the node 103 and node 104 is blocked but the link between the node 101 and node 106 is reconnected. Such a change will cause PTP/SyncE system re-constructing, which is time consuming according to the current protocols. For example, the master-slave relationship and the SyncE clock chain are built up through SyncE initiative message exchanging, which is a layer 2 message type, while SyncE is a layer 1 fast clock signal. Due to slow detection of synchronization loss by Ethernet Synchronization Message Channel (ESMC), it typically takes several seconds to re-establish the SyncE clock chain. Similarly, for PTP, the Master/Slave relationship and PTP hierarchy are built up through PTP initiative messages exchanging, and the PTP port in the PTP hierarchy uses its slow Announce message, which is typically 1 packet in 2 seconds, to detect a synchronization loss. As a result, a node loosing sync due to a network failure may take minutes to re-establish synchronization. The slow clock recovery in the network leads to frequency and time shift and an undesirable long network downtime.